1. Field of the Invention
The present invention relates to a multi-domain vertical alignment (MVA) liquid crystal display device having a plurality of regions (domains) in which the orientations of liquid crystal molecules are different from each other. In particular, the present invention relates to a liquid crystal display device in which a picture element electrode is divided into a plurality of sub picture element electrodes.
2. Description of the Prior Art
Liquid crystal display devices have the advantages that they are thin and light compared to cathode-ray tube (CRT) displays and that they can be operative at low voltages and have low power consumption. Accordingly, liquid crystal display devices are used in various kinds of electronic devices including televisions, notebook personal computers (PCs), desktop PCs, personal digital assistants (PDAs), and mobile phones. In particular, active matrix liquid crystal display devices in which a thin film transistor (TFT) as a switching element is provided for each picture element (sub-pixel) show excellent display characteristics, which are comparable to those of CRT displays, because of high operation capabilities thereof, and therefore have come to be widely used even in fields where CRT displays have been used heretofore, such as desktop PCs and televisions.
In general, as shown in FIG. 1, a liquid crystal display device includes: two transparent substrates 10 and 20 which are placed with spacers 31 interposed therebetween and which are bonded together using a sealing member 32; and liquid crystals 30 contained between the substrates 10 and 20. On one substrate 10, a picture element electrode, a TFT, and the like are formed for each picture element. On the other substrate 20, color filters facing the picture element electrodes and a common electrode, which is common to the picture elements, are formed. The color filters are classified into three types of red (R), green (G), and blue (B). A color filter of any one color is placed in each picture element. Three picture elements of red (R), green (G), and blue (B) which are adjacently placed constitute one pixel.
Hereinafter, the substrate on which the picture element electrodes and the TFTs are formed is referred to as a TFT substrate, and the substrate placed to face the TFT substrate is referred to as a counter substrate. Further, the structure formed by filling the liquid crystals into the space between the TFT substrate and the counter substrate is referred to as a liquid crystal panel.
The TFT substrate 10 is formed to be larger than the counter substrate 20 by an amount corresponding to connection terminals. Polarizing plates 41 and 42 are placed on both sides of the liquid crystal panel 40 including the TFT substrate 10 and the counter substrate 20, respectively. Moreover, a backlight (not shown) is placed under the liquid crystal panel 40.
Heretofore, twisted nematic (TN) liquid crystal display devices have been widely used in which horizontal alignment-type liquid crystals (liquid crystals with positive dielectric anisotropy) are contained between two substrates 10 and 20 and in which the liquid crystal molecules are twisted and aligned. However, TN liquid crystal display devices have the disadvantage that viewing angle characteristics are poor and that contrast and color greatly change when the screen is viewed from an oblique direction. Accordingly, vertical alignment (VA) liquid crystal display devices and multi-domain vertical alignment (MVA) liquid crystal display devices, which have favorable viewing angle characteristics, have been developed and put into practical use.
FIGS. 2A and 2B are cross-sectional schematic views showing one example of an MVA liquid crystal display device. A TFT substrate 10 and a counter substrate 20 are placed with spacers (not shown) interposed therebetween, and vertical alignment-type liquid crystals (liquid crystals with negative dielectric anisotropy) 30 are contained between these substrates 10 and 20. On a picture element electrode 12 of the TFT substrate 10, a plurality of bank-like protrusions 13 are formed as domain regulation structures. The surfaces of the picture element electrode 12 and the protrusions 13 are covered with a vertical alignment film 14 made of, for example, polyimide.
A plurality of bank-like protrusions 23 are also formed as domain regulation structures under a common electrode 22 of the counter substrate 20. These protrusions 23 are placed at positions obliquely deviated from the protrusions 13 on the substrate 10. The surfaces of the common electrode 22 and the protrusions 23 are also covered with a vertical alignment film 24 made of, for example, polyimide.
In the MVA liquid crystal display device, in the state where a voltage is not applied between the picture element electrode 12 and the common electrode 22, most of the liquid crystal molecules 30a are aligned perpendicular to the substrate surfaces as shown in FIG. 2A. However, the liquid crystal molecules 30a in the vicinities of the protrusions 13 and 23 are aligned with directions perpendicular to the inclined surfaces of the protrusions 13 and 23.
When a predetermined voltage is applied between the picture element electrode 12 and the common electrode 22, the liquid crystal molecules 30a are aligned with an oblique direction relative to the substrate surfaces under the influence of an electric field. In this case, as shown in FIG. 2B, the tilt directions of the liquid crystal molecules 30a are different on opposite sides of each of the protrusions 13 and 23, and so-called domain division (multi-domain) is achieved.
As shown in this FIG. 2B, in the MVA liquid crystal display device, the tilt directions of the liquid crystal molecules 30a when a voltage is applied are different on opposite sides of each of the protrusions 13 and 23. Accordingly, the leakage of light in oblique directions is suppressed, and excellent viewing angle characteristics can be obtained.
Although the case where domain regulation structures are protrusions has been described in the above-described example, slits provided in electrodes or dimples (grooves) in a substrate surface are used as domain regulation structures in some cases. Further, though an example in which domain regulation structures are provided on both of the TFT substrate 10 and the counter substrate 20 has been described in FIGS. 2A and 2B, domain regulation structures may be formed only on any one of the TFT substrate 10 and the counter substrate 20.
FIG. 3 shows an example in which slits 12a are formed as domain regulation structures in the picture element electrode 12 on the TFT substrate 10. Since electric flux lines occur in oblique directions in the vicinities of the edge portions of the slits 12a, the tilt directions of the liquid crystal molecules 30a are different on opposite sides of each slit 12a. Thus, alignment division is achieved, and viewing angle characteristics are improved.
FIG. 4 is a plan view showing one picture element of an actual MVA liquid crystal display device, and FIG. 5 is a cross-sectional schematic view of a TFT substrate of the same liquid crystal display device.
On the TFT substrate 50, a plurality of gate bus lines 51 extending horizontally and a plurality of data bus lines 55 extending vertically are placed with predetermined pitches, respectively. Each of the rectangular areas defined by the gate bus lines 51 and the data bus lines 55 is a picture element region. Further, auxiliary capacitance bus lines 52 which are placed parallel to the gate bus lines 51 and which cross the centers of the picture element regions are formed on the TFT substrate 50. A first insulating film 61 is formed between the gate bus lines 51 and the data bus lines 55 and between the auxiliary capacitance bus lines 52 and the data bus lines 55. The gate bus lines 51 and the auxiliary capacitance bus lines 52 are electrically isolated from the data bus lines 55 by the first insulating film 61.
For each picture element region, a TFT 54, a picture element electrode 56, and an auxiliary capacitance electrode 53 are formed. The TFT 54 uses part of the gate bus line 51 as a gate electrode. Further, the drain electrode 54d of the TFT 54 is connected to the data bus line 55, and the source electrode 54s thereof is formed at a position where the source electrode 54s faces the drain electrode 54d across the gate bus line 51. Furthermore, the auxiliary capacitance electrode 53 is formed at a position where the auxiliary capacitance electrode 53 faces the auxiliary capacitance bus line 52 with the first insulating film 61 interposed therebetween.
The auxiliary capacitance electrode 53, the TFT 54, and the data bus line 55 are covered with a second insulating film 62, and the picture element electrode 56 is placed on the second insulating film 62. The picture element electrode 56 is made of transparent conductive material such as indium-tin oxide (ITO) or the like and electrically connected to the source electrode 54s of the TFT 54 and the auxiliary capacitance electrode 53 through contact holes 62a and 62b formed in the second insulating film 62. Further, two slits 56a extending diagonally are horizontal-line symmetric, in the picture element electrode 56. The surface of the picture element electrode 56 is covered with a vertical alignment film (not shown) made of, for example, polyimide.
On a counter substrate placed to face the TFT substrate 50, a black matrix (light blocking film), color filters, and a common electrode are formed. As represented by dot-dashed lines in FIG. 4, a plurality of bank-like protrusions 71 bending at positions over the gate bus lines 51 and the auxiliary capacitance bus lines 52 are formed on the common electrode. The slits 56a of the picture element electrodes 56 are placed between the protrusions 71.
In the liquid crystal display device formed as described above, when a predetermined voltage is applied between the picture element electrode 56 and the common electrode, four domains A1, A2, A3, and A4 in which the orientations of liquid crystal molecules 30 are different from each other are formed as shown in FIG. 6. These domains A1, A2, A3, and A4 are separated by the protrusions 71 and the slits 56a as boundaries. In the case where the slits 56a and the protrusions 71 are formed so that the areas of the domains A1, A2, A3, and A4 become approximately equal to each other, the direction dependency of viewing angle characteristics becomes small.
Incidentally, in a known MVA liquid crystal display device, the phenomenon occurs in which the screen looks whitish when viewed from an oblique direction. FIG. 7 is a view showing T-V (transmittance-voltage) characteristics for the case where the screen is viewed from the front and those for the case where the screen is viewed from above in a direction of 60°, with applied voltage (V) on the horizontal axis and transmittance on the vertical axis. As shown in this FIG. 7, in the case where a voltage slightly higher than a threshold voltage is applied to the picture element electrode (portion circled in the drawing), the transmittance when the screen is viewed from the oblique direction is higher than that when the screen is viewed from the front. Further, when the applied voltage becomes high to some extent, the transmittance when the screen is viewed from the oblique direction becomes lower than that when the screen is viewed from the front. Accordingly, differences in brightness between red, green, and blue picture elements become small when the screen is viewed from the oblique direction. As a result, the phenomenon in which the screen looks whitish occurs as described previously. This phenomenon is called discolor. Discolor occurs not only in MVA liquid crystal display devices but also in TN liquid crystal display devices.
In the specification of U.S. Pat. No. 4,840,460, a technology is proposed in which each picture element is divided into a plurality of sub picture elements and in which these sub picture elements are capacitively coupled. In such a liquid crystal display device, since a potential is divided in accordance with the capacitance ratio between the sub picture elements, different voltages can be applied to the sub picture elements, respectively. Accordingly, it appears that a plurality of regions having different thresholds of T-V characteristics exist in each picture element. In the case where a plurality of regions having different thresholds of T-V characteristics exist in each picture element as described above, the phenomenon is suppressed in which the transmittance when the screen is viewed from an oblique direction becomes higher than that when the screen is viewed from the front as shown in FIG. 7. As a result, the phenomenon (discolor) in which the screen looks whitish is also suppressed. A method in which display characteristics are improved by dividing each picture element into a plurality of sub picture elements capacitively coupled is called a halftone grayscale (HT) method by capacitive coupling.
In the specification of Japanese Patent No. 3076938 (Japanese Unexamined Patent Publication No. Hei 5(1993)-66412), a liquid crystal display device is disclosed in which each picture element electrode is divided into a plurality (four in FIG. 8) of sub picture element electrodes 81a to 81d and in which control electrodes 82a to 82d are respectively placed under the sub picture element electrodes 81a to 81d with an insulating film interposed therebetween, as shown in FIG. 8. In this liquid crystal display device, the sizes of the control electrodes 82a to 82d are different from each other, and a display voltage is applied to the control electrodes 82a to 82d through a TFT 80. Further, in order to prevent the leakage of light from between the sub picture element electrodes 81a to 81d, a control electrode 83 is also placed between the sub picture element electrodes 81a to 81d. 
In the specification of Japanese Patent No. 3401049 (Japanese Unexamined Patent Publication No. Hei 6(1994)-332009), a liquid crystal display device is disclosed in which each picture element is divided into a plurality of sub picture elements. In this liquid crystal display device, the pre-tilted angles of liquid crystal molecules at the surfaces of the sub picture elements are made to be different from each other by, for example, changing rubbing conditions for each sub picture element.
Each of these known technologies relates to a TN liquid crystal display device.
Incidentally, in the HT method by capacitive coupling, the dividing of each picture element into a plurality of sub picture elements generates gaps between the sub picture elements, and an aperture ratio greatly decreases. In a typical TN liquid crystal display device of the normally white mode, the gaps between sub picture elements become portions in which transmittance is high. Accordingly, a black matrix for blocking light in the gaps between the sub picture elements needs to be formed on a counter substrate. However, in view of the prevention of misalignment in bonding a TFT substrate and the counter substrate together and the prevention of light leakage in oblique directions, the widths of the black matrix need to be set larger than those of the gaps between the sub picture elements by approximately 20 μm (approximately 10 μm on one side). This causes a significant decrease in the aperture ratio.
As described in the specification of Japanese Patent No. 3076938, it is also possible to control the transmittance by forming control electrodes even in the gaps between the sub picture elements. However, in this case, both the control electrodes and the sub picture element electrodes need to be formed of transparent conductive material such as ITO or the like. This requires two steps for depositing transparent conductive material films and two steps for photolithography, and causes an increase in manufacturing cost.
In a liquid crystal display device described in the specification of Japanese Patent No. 3401049, the pre-tilted angles of liquid crystal molecules are made to be different in each sub picture element by, for example, changing rubbing conditions for each sub picture element. However, dust occurring in rubbing can come to be mixed in liquid crystals to deteriorate display quality. Thus, the advantage of MVA liquid crystal display devices that rubbing is unnecessary is lost.
Moreover, in MVA liquid crystal display devices, the pre-tilted angles of liquid crystal molecules need to be stably aligned in a very narrow range of approximately 88 to 89° in order to realize alignment division. For example, if the pre-tilted angles of the liquid crystal molecules become 86° or less, light passes when a voltage is not applied, and contrast decreases; if the pretilt angles become 89.5° or more, the liquid crystal molecules do not easily tilt in predetermined directions when a voltage is applied. However, it is very difficult to control the pretilt angles of the liquid crystal molecules in a range of approximately 88 to 89° with high precision by rubbing. Further, the pretilt angles of the liquid crystal molecules after rubbing has been performed on a vertical alignment film have very poor stability and easily change in water washing and heat treatment thereafter.